SnO2 ISFET device, manufacturing method, and methods and apparatus for use thereof

ABSTRACT

A SnO 2  ISFET device and manufacturing method thereof. The present invention prepares SnO 2  as the detection membrane of an ISFET by sol-gel technology to obtain a SnO 2  ISFET. The present invention also measures the current-voltage curve for different pH and temperatures by a current measuring system. The temperature parameter of the SnO 2  ISFET is calculated according to the relationship between the current-voltage curve and temperature. In addition, the drift rate of the SnO 2  ISFET for different pH and hysteresis width of the SnO 2  ISFET for different pH loop are calculated by a constant voltage/current circuit and a voltage-time recorder to measure the gate voltage of the SnO 2  ISFET.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to an ISFET, and in particular to aSnO₂ ISFET device, manufacturing method, and method and apparatus tomeasure hysteresis width and drift rate therewith.

[0003] 2. Description of the Related Art

[0004] ISFETs (Ion Sensitive Field Effect Transistor) are constructed bysubstituting a detecting film for the metal gate on the gate oxide of atraditional MOSFET (Metal-Oxide-Semiconductor Field Effect Transistor).When the ISFET is dipped into a solution, the interfacial potentialbetween the detecting film and the solution influences the semiconductorsurface since only an extremely thin dielectric (that is, the gateoxide) separates the detecting film from the semiconductor surface. Thisinfluences the charge density in the inversion layer of thesemiconductor surface, and thereby modulates the channel current throughthe ISFET. Thus, by utilizing this characteristic, the pH or other ionconcentration in a solution can be obtained from the measurement ofsource/drain current and the gate voltage of the ISFET. The potentialdifference on the interface between the detecting film and the solutionis related to the ion activity in a solution. The hydrogen ion activityin the solution can be measured using different channel currents causedby different interfacial potential differences in various solutions withdifferent hydrogen ion activity.

[0005] Patents related to the formation of the ISFET or measurementthereof are listed hereinafter.

[0006] U.S. Pat. No. 5,350,701 discloses a method of measuring thecontent of alkaline-group metals, especially the content of the calciumions, utilizing chemosynthesis phosphide group as the detecting film ona gate of an ISFET.

[0007] U.S. Pat. No. 5,387,328 discloses a bio-sensor using ionsensitive field effect transistor with platinum, wherein an enzymemembrane is immobilized on the ion-detecting film to determine theconcentration of glucose.

[0008] U.S. Pat. No. 5,414,284 discloses a method of fabricating anISFET and an ESD protective circuit on the same silicon substrate,wherein a capacitor is utilized as an interface between the protectivecircuit and a sample solution to the DC leakage current.

[0009] U.S. Pat. No. 5,309,085 integrates the measurement circuit of acreature sensor having ISFET on a wafer. The measured circuit has twoISFET devices, an enzyme ISFET and a reference electrode FET, whoseoutput signal can be amplified by a differential amplifier.

[0010] U.S. Pat. No. 5,061,976 discloses a carbon thin film on the gateoxide of the ISFET and then a 2,6 xylenol electrolytic polymerizationfilm formed thereon. The ISFET has the ability to detect hydrogen ionsand the advantages of small floating time, high reliability, andinsensitivity to light. When other film types are covered on the ISFET,other kinds of ions can be detected.

[0011] U.S. Pat. No. 5,833,824 discloses an ISFET sensor for detectingion activity in a solution, which includes a substrate and an ISFETsemiconductor die. The substrate has a front surface exposed to thesolution, a back surface opposite to the front surface and an apertureextending therebetween. A detecting film of the ISFET is mounted on theback surface such that the gate region is exposed to the solutionthrough the aperture.

[0012] U.S. Pat. No. 4,691,167 discloses a method of measuring ionactivation in a solution by combining the ISFET, the referenceelectrode, the temperature sensor, amplifier circuit and a calculationand memory circuit. Since the sensitivity is a function of thetemperature and drain current of ISFET and is decided by a variable ofgate voltage, the sensitivity can be obtained by calculating formulasstored in memory.

[0013] U.S. Pat. No. 5,130,265 discloses a method of fabricating theISFET with multiple functions. The method uses siloxanic prepolymer asthe sensitive film, mixing the solution, photochemistry treatment andheat treatment.

[0014] U.S. Pat. No. 4,660,063 discloses a method of performing bothlaser drilling and solid diffusion to form a 3D diode array on thesemiconductor wafer. The laser first drills the wafer, and theimpurities are then diffused from the hole to form a cylindrical PNjunction and complete a non-planar ISFET structure.

[0015] U.S. Pat. No. 4,812,220 discloses an ISFET made by fixing theenzyme on the detecting film to measure the concentration of amino acidsin food. The enzyme sensor is miniaturized, and can accurately measureconcentrations, even when small.

[0016] There are many materials acting as detection membranes of ISFETs,such as, Al₂O₃, Si₃N₄, Ta₂O₅, a-WO₃, a-Si:H and the like. These thinfilms are deposited by either sputtering or plasma enhanced chemicalvapor deposition (PECVD), therefore, the cost of the thin filmfabrication is higher. For commercial purpose, it is important todevelop a thin film, with low cost and easy fabrication.

[0017] In the ISFET applications, however, many factors such ashysteresis, temperature, and drift behavior affect the accuracy of themeasuring results. Since pH-ISFET is a semiconductor device, it iseasily influenced by variations in temperature. The variation of thetemperature leads to a deviation of the measurement. With reference tothe hysteresis behavior, it is related to the change in the pH of thesolution (such as pH_(x)→pH_(y)→pH_(x)→pH_(z)→pH_(x)) and thecorresponding change in the output voltage of the ISFET (such asV_(ox1)→V_(oy)→V_(ox2)→V_(oz)→V_(ox3)). At the same pH, the differencebetween the first output voltage and the final output voltage (such asV_(ox3)−V_(ox1)) is defined as the hysteresis width. For drift behavior,the drift rate is defined as the change in the gate voltage per unittime under conditions in which the source-drain current is stable andthe temperature is constant after the intrinsic response of the pH-ISFETis completed. Hence, there is a need to measure the three effects toprevent error.

SUMMARY OF THE INVENTION

[0018] In view of this, an object of the invention is to provide a SnO₂gate ISFET. The invention forms the SnO₂ layer as the detection membraneof the ISFET by sol-gel technology.

[0019] Another object of the invention is to provide a method ofmeasuring temperature parameters of an ISFET. In the present invention,the sensitivities of the ISFET at different temperatures are obtained bythe source-drain current and gate voltage of the ISFET in a solution,and temperature parameters (temperature coefficient of the sensitivity)of an ISFET are further obtained.

[0020] In the method of measuring the temperature parameters of an ISFETaccording to the present invention, the detecting film is immersed in abuffer solution, and, then, at a predetermined temperature, the pH ofthe buffer solution is changed to measure and record the source-draincurrent and the gate voltage of the ISFET to obtain a curve. Thetemperature parameters at the predetermined temperature are obtained byselecting a fixed current from the curve. The temperature parameters atother temperatures are obtained by changing the temperature of thebuffer solution and the steps of measuring, recording and selecting.

[0021] Another object of the present invention is to provide a method ofmeasuring the hysteresis width and drift rate of the SnO₂ ISFET to usethe reverse compensation method to obtain the accurate output value.

[0022] In the method of measuring the hysteresis width of a SnO₂ ISFETaccording to the present invention, first, the drain-source current andthen the drain-source voltage are fixed by a constant voltage/currentcircuit, and the SnO₂ ISFET is immersed in a buffer solution. Thegate-source output voltage of the SnO₂ ISFET is recorded by avoltage-time recorder, and the pH of the buffer solution is changed. Thesteps of immersing and recording are then repeated to obtain thegate-source output voltages of the ISFET immersed in the buffer solutionwith different pH. The hysteresis width is the voltage deviation betweenstarting pH and ending pH.

[0023] In the method of measuring the drift rate of a SnO₂ ISFETaccording to the present invention, first, the drain-source current andthen the drain-source voltage are fixed by a constant voltage/currentcircuit, and the SnO₂ ISFET is immersed in a buffer solution. Thegate/source output voltage of the SnO₂ ISFET during a constant period isrecorded by a voltage recorder. The pH of the buffer solution is changedand the steps of immersing and recording are repeated to obtain thegate-source output voltages of the ISFET immersed in the buffer solutionwith different pH. The drift rate is the slope of the gate-source outputvoltage with respect to time.

[0024] Another object of the present invention is to provide anapparatus to measure the hysteresis width and the drift rate. Theapparatus of measuring the hysteresis width and the drift rate has aSnO₂ ISFET, a buffer solution to contact the ISFET, a light-isolationcontainer to load the buffer and to isolate light, a heater to heat thebuffer solution, a constant current/voltage measuring device coupled tothe source and drain of the SnO₂ ISFET, and a voltage-time recorder torecord the output voltage of ISFET.

[0025] A detailed description is given in the following embodiments withreference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0026] The present invention can be more fully understood by reading thesubsequent detailed description and examples with references made to theaccompanying drawings, wherein:

[0027]FIGS. 1a to 1 c are cross-sections of the processes according tothe present invention;

[0028]FIG. 2 is a cross-section of an ISFET with gate consisting ofSnO₂;

[0029]FIG. 3 is a schematic structural diagram of the apparatus tomeasure the temperature parameter of the SnO₂ ISFET according to thepresent invention;

[0030]FIG. 4 is a schematic cross-section of the SnO₂ ISFET according tothe present invention;

[0031]FIG. 5 shows a curve related to the source/drain current and thegate voltage of the SnO₂ ISFET at 25° C. according to the presentinvention;

[0032]FIG. 6 shows a curve related to the gate voltage of the SnO₂ ISFETand the pH;

[0033]FIG. 7 shows a curve related to the sensitivities of the SnO₂ISFET and the temperature according to the present invention;

[0034]FIG. 8 shows a schematic structure diagram of the apparatus tomeasure the hysteresis width and drift rate of the SnO₂ ISFET accordingto the present invention;

[0035]FIG. 9 shows a schematic diagram of the constant voltage currentcircuit according to the present invention;

[0036]FIG. 10 shows the relationship between the hysteresis width andtime in pH=5→1→5→9→5 order;

[0037]FIG. 11 shows the relationship between the hysteresis width andtime in pH=4→1→4→7→4 order; and

[0038]FIG. 12 shows the relationship between the drift rate of the SnO₂and the pH.

DETAILED DESCRIPTION OF THE INVENTION

[0039] The SnO₂ gate ion sensitive field effect transistor (SnO₂ gateISFET) according to the present invention is illustrated in FIGS. 1a to1 c.

[0040] As shown in FIG. 1a, a P-type (100) semiconductor substrate 100with a resistivity ranging from 8 to 12 Ω-cm is provided. A pad oxidelayer 102 consisting of silicon dioxide with a thickness of 5000 Å isformed on the substrate 100 by wet oxidation. A first photoresistpattern (not shown) is formed on the pad oxide layer 102 by conventionalphotolithography. A dummy gate 103 is formed to define the subsequentgate area, using the photoresist pattern as a mask to a portion of thepad oxide layer 102.

[0041] Impurities are then implanted into the semiconductor substrate100 to form a source/drain 104 beside the dummy gate 103, using thedummy gate 103 as a mask. For example, the impurities implanted hereinare boron ions with a dose of 10¹⁵ cm⁻².

[0042] As shown in FIG. 1b, the dummy gate 103 is removed, namely, thepad oxide layer 102 and the first photoresist pattern are removed by wetetching. An insulating layer 106 consisting of silicon dioxide with athickness of about 1000 Å is then formed on the semiconductor substrate100. A second photoresist pattern (not shown) is formed on theinsulating layer 106 by photolithography. Next, the insulating layer 106outside the gate area is removed by the second photoresist pattern as amask. The residual insulating layer within the gate area is used as agate oxide layer. Subsequently, the second photoresist layer is removed.

[0043] A SnO₂ layer 108 is then formed on the insulating layer 106 bysol-gel technology. For example, the SnO₂ layer 108 with a thickness ofleast 1000 Å is formed on the insulating layer 106. A two-layer gateconsisting of the gate oxide layer 106 and the SnO₂ layer 108 isfabricated. Thus, a SnO₂ ISFET is obtained. The SnO₂ ISFET has a channellength of about 50 μm and a channel width of about 1000 μm. Thus, theaspect ratio (channel width/channel length) of the present SnO₂ ISFET is20.

[0044] Next, as shown in FIG. 1c, an interconnecting process isperformed to obtain the ion sensitive field effect transistor (ISFET)using conventional interconnect steps for MOS. Thus, an insulating layer100 is formed on the source/drain 104, and a metal wire 112 is formed onthe insulating 110 by etching and sputtering. Finally, a sealing layer114 consisting of the insulator is formed to seal the metal wireexcluding the SnO₂ layer 108. For example, the metal wires 112 can bealuminum, and the sealing layer 114 can be epoxy resin.

[0045] The step of forming the SnO₂ layer 108 on the gate oxide layer106 by sol-gel technology is illustrated as follows. First, a mixingsolution consisting of SnCl₂.2H₂O and ethanol is prepared at aconcentration of about 0.37M. The mixing solution is then coated on thesurface of the gate oxide layer 106 of the ISFET. Finally, the ISFETcoated with mixing solution is heated at a temperature of about 350° C.to 400° C. for one hour. After that, the SnO₂ gate layer 108 is obtainedon the oxide layer 106 for the SnO₂ ISFET.

[0046] The use of sol-gel technology to form the SnO₂ detection membraneprovides a low temperature process using less equipment at a lower cost,and enables formation of a thin, uniform detection membrane. Additionalbenefits also include easily formed SnO₂ detection membrane on a largearea surface of the substrate, easy modification of the components ofthe sol-gel to produce the desired detection membrane, and more easilycontrolled accuracy in microstructure of the detection membrane, such asporosity, aperture, and specific surface.

[0047]FIG. 2 is a cross-section of the SnO₂ ISFET according to thepresent invention. The structure of this ISFET is similar to that ofMOSFET. The difference between the ISFET and MOSFET is that the metalgate of the MOSFET is replaced by a SnO₂ detection membrane 35, anaqueous solution 36 and reference electrode 38. The circuits are formedby contacting the metal wire 31, preferably consisting of Al, with thesource/drain 33. Since the SnO₂ detection membrane 35 contacts thedetected solution 36, the whole device in addition to the SnO₂ detectionmembrane 35 must be enclosed by a sealing layer 37 consisting of amaterial with good insulating property, such as epoxide resin. Thereference electrode 38 provides a detecting base.

[0048] The SnO₂ detection membrane 35 of the ISFET is immersed in thesolution 36 during operation, such that the point of transformation fromchemical equivalence into electrical equivalence within the ISFET occurswith contact between the SnO₂ detection membrane 35 and the aqueoussolution 36. The reaction mechanism of the ionic activity within thesolution is the interface potential obtained from the interface betweenthe aqueous solution 36 and the SnO₂ detection membrane 35 immersed inthe aqueous solution 36. The interface potential varies with the ionicactivities of various aqueous solutions. In addition, the interfacepotential regulates the channel conduction of the ISFET and results inthe change of current within the source/drain 33.

[0049]FIG. 3 shows a systematic structure diagram according to theinvention. An ISFET using SnO₂ as a detection membrane (called SnO₂ISFET) is immersed in a buffer solution 2 such as the phosphate buffersolution that is stored in a container (not titled). The source/drain(not shown) of the SnO₂ ISFET 1 connects a test fixer 3 through twoconnecting wires 331, respectively, to convey the electrical signalobtained by measuring the source/drain to a current/voltage measuringdevice 4, such as the Keithley-236 current/voltage measuring device fordata processing.

[0050] Also, a reference electrode 5 is disposed in the buffer solution2, with one end connected to the test fixer 3 through the connectingwire 331. A plurality of heater 6 is disposed outside the container andconnected to a PID temperature controller 7. A thermometer 8 connectedto the PID temperature controller 7 detects the temperature of thebuffer solution 2. The above-mentioned elements such as the buffersolution 2, the elements contacting the buffer solution 2 and the heater6 are placed in a light-isolating container 9 to prevent the measuringdata from the effect of light.

[0051] It should be noted that the interfacial potential between theSnO₂ membrane and the solution, and the characteristic difference ofcharge density in the reverse layer of the semiconductor surface is usedto measure needed data (such as the source-drain current or the gatevoltage) and thus obtain the temperature parameters of the ISFET.

[0052]FIG. 4 is a schematic cross-section of the SnO₂ ISFET according tothe present invention. The SnO₂ ISFET is formed on a semiconductorsubstrate 20 such as a p-type silicon substrate. A pair of source/drainregions 21 separated from each other are formed approaching the topsurface of the semiconductor substrate 20 and each region 21 isconnected to the test fixer outside through an aluminum contact plug 22and an aluminum wire 23. On the semiconductor substrate 20 between thetwo drain/source regions 21, a gate oxide 24, like a silicon oxidelayer, and a SnO₂ detection membrane 25 are formed sequentially. Anepoxy resin 26 seals the device but exposes the SnO₂ detection membrane25. As well, a metal-aluminum layer 27 is formed at the bottom of thesemiconductor substrate 20 to shield light and decrease its influence oncharge carriers.

[0053] With reference to apparatus shown in FIG. 3 and FIG. 4, thetemperature parameters of the SnO₂ ISFET of the present invention can beobtained according to a method as follows. First, with regard to themeasurement of the sensitivity, the detection membrane of the SnO₂ ISFETcontacts with the buffer solution. The temperature of the buffersolution is fixed, such as at 25° C., and the pH of the buffer solutionis changed at the same time. The curve related to the source-draincurrent and the gate voltage of the SnO₂ ISFET is measured and recordedby the Keithley-236 current/voltage measuring device. FIG. 5 showscurves related to the statistical result, and both the source-draincurrent and the gate voltage of the SnO₂ ISFET rise as the pH of thebuffer resolution increases.

[0054] Next, a fixed current of the curve (like 80 μA) is selected toobtain a curve related to the gate voltage and the pH at a specifictemperature (like 25° C.) as shown in FIG. 6, wherein the sensitivity ofthe SnO₂ ISFET at 25° C. is 57.36 mV/pH. It is found that the gatevoltage of the SnO₂ ISFET is in direct proportion to the pH of thebuffer solution and the slope of the curve is the sensitivity of theSnO₂ ISFET at the specific temperature.

[0055] Moreover, in order to measure the sensitivity of the SnO₂ ISFETat different temperatures, only the temperature of the buffer solutionneeds to be changed, such as between 15° C.-45° C., with the above steprepeated at each temperature. Table 1 shows the sensitivity of the ISFETat the different temperatures. TABLE 1 the sensitivity of the ISFET atthe different temperatures Temperature (° C.) 15 25 35 45 Sensitivity55.1 57.16 58.66 60.36 (mV/pH)

[0056] A curve showing the obtained sensitivities at differenttemperatures is shown in FIG. 7, wherein the sensitivity is in directproportion to the rising temperature and the slope of the curve is about0.173 mV/pH° C. Namely, the temperature parameter of the SnO₂ ISFET isabout 0.173 mV/pH° C.

[0057]FIG. 8 shows a schematic diagram to measure the hysteresis widthand the drift rate of an ISFET with SnO₂ as a detection membraneaccording to present invention. An ISFET 81 with SnO₂ as a detectionmembrane (called SnO₂ ISFET) is immersed in a buffer solution 82 such asa standard buffer solution and carried by a container (not labeled). Adrain/source (not shown) of the SnO₂ ISFET 81 is connected to a constantvoltage/current circuit 83 (such as a negative feedback circuit) throughtwo wires 811 and 812. The drain-source voltage and the drain-sourcecurrent of the SnO₂ ISFET 81 are fixed by the constant voltage/currentcircuit 83.

[0058] A reference electrode 84 is disposed in the buffer solution 82,wherein one end of the reference electrode 84 is connected to theconstant voltage/current circuit 83 through a wire 841. A heater 85disposed outside the container is connected to a PID(Proportional-Integral-Derivative) temperature controller 86. Both theheater 85 and the PID temperature controller 86 keep the buffer solution82 at a constant temperature (preferably 25° C.) detected by athermocouple 87 connected to the PID temperature controller 86. Theabove buffer solution 82, every device connected thereto, and the heater86 are disposed in a light-isolating container 88 to reduce the effectof light on the measuring results.

[0059] The constant voltage/current circuit 83 is connected to acurrent/voltage measuring device 89 composed of two digital multimetersdetecting whether the source-drain current and the source-drain voltageof the SnO₂ ISFET 81 move towards stability. Also, the constantvoltage/current circuit 83 is connected to a voltage-time recorder 810for setting and recording the output voltages during each recordingperiod.

[0060]FIG. 9 shows a schematic diagram of the constant voltage/currentcircuit 83 according to the present invention. The constantvoltage/current circuit 83 is connected to the source/drain of the SnO₂ISFET 81 through the wires 811 and 812, and is connected to thereference electrode 84 through the wire 841. The source-drain voltage isfixed at a constant value (preferably 0.2V) by adjusting the variableresistance R1. The source-drain current is fixed at a constant value(preferably 50 μA). In this case of the negative feedback circuit, theoutput voltage and the gate voltage are reduced and finally thedrain-source current I_(DS) is reduced when the increasing drain-sourcecurrent I_(DS) increases the source voltage. The circuit 83 hasadvantages of simplicity, low cost, ease of operation and no need foradjustment of the measuring point.

[0061] Next, returning to FIG. 4, a schematic cross-section of the SnO₂ISFET according to the present invention, the SnO₂ ISFET is formed on asemiconductor substrate 20 such as a p-type silicon substrate. In thiscase, a pair of source/drain regions 21 separated from each other areformed approaching the top surface of the semiconductor substrate 20,and are connected to constant voltage/current circuit by an aluminumcontact plug 22 and an aluminum wire 23. On the semiconductor substrate20 between the two drain/source regions 21, a gate oxide 24, such as asilicon oxide layer, and a SnO₂ detection membrane 25 are formedsequentially. An epoxy resin 26 seals the device but exposes the SnO₂detection membrane 25. As well, a metal-aluminum layer 27 is formed atthe bottom of the semiconductor substrate 20 to decrease thechannel-adjusting effect.

[0062] Hereinafter, a method to measure the hysteresis width and thedrift rate of the SnO₂ ISFET in detail is described with reference toFIGS. 2, 8 and 9.

[0063] With reference to the method of measuring the hysteresis width ofthe SnO₂ ISFET, first, the drain-source current and the drain-sourcevoltage of the SnO₂ ISFET 81 are fixed by the constant voltage/currentcircuit (negative feedback circuit) 83. In this step, the SnO₂ ISFET andthe reference electrode are connected to the constant voltage/currentcircuit 83, and are immersed in the solution. Next, the drain voltageV_(D) of the SnO₂ ISFET 81 is set at 0.2V by adjusting the variableresistant R1 and measuring by one digital multimeter. Also, thedrain-source current IDS is set at 50 μA by adjusting the variableresistant R2 and measuring by the other digital multimeter. The SnO₂ISFET 81 is then placed in a standard solution to maintain stability.

[0064] After, the SnO₂ ISFET 81 is immersed in a buffer solution. Next,the voltage-time recorder records the gate-source output voltages ofSnO₂ ISFET 81. The hysteresis width of the SnO₂ ISFET in accordance withthe pH of the buffer solution is then measured. In the presentinvention, hysteresis width is measured in pH=5→1→5→9→5 order, namelypH=5→4→3→2→1→2→3→4→5→6→7→8→9→8→7→6→5, wherein each measuring result isachieved at the time the pH is changed for one minute, the loop time is1020 seconds, and one time the pH changes one unit. Particularly,choosing the pH=5→1→5→9→5 order measures hysteresis within the pH rangebetween 1 and 9. Also, the hystereses within the pH range between 1 and9 are measured in the same way, wherein each result is at the time whenthe pH is changed for two minutes and four minutes, the loop time is2040 seconds and 4080 seconds. Table 2 shows the hysteresis width of theSnO₂ ISFET in pH=5→1→5→9→5 order at different loop time. By the samemethod, all of the hysteresis width at different pH values can bemeasured, helpful in performing the reverse compensation method. Table 3shows the hysteresis width of the SnO₂ ISFET in pH=4→1→4→7→4 order atdifferent loop time. TABLE 2 the hysteresis width of the SnO₂ ISFET inpH = 5→1→5→9→5 order at different loop time Loop time (seconds)Hysteresis width (mV) 1020 3.74 2040 4.79 4080 5.46

[0065] TABLE 3 the hysteresis width of the SnO₂ ISFET in pH = 4→1→4→7→4order at different loop time Loop time (seconds) Hysteresis width (mV)980 1.3 1960 2.85 3920 4.47

[0066] The relationship between the hysteresis width, the pH and timemeasured in pH=5→1→5→9→5 order is shown in FIG. 10. Also therelationships of the hysteresis width, the pH and time measured atpH=4→1→4→7→4 order is shown in FIG. 11. As shown in FIGS. 10 and 11, thehysteresis width of the SnO₂ ISFET increases as the loop time increases.

[0067] Next, with reference to the drift rate, the drain-source currentand the drain-source voltage of the SnO₂ ISFET is fixed by the constantvoltage/current circuit (negative feedback circuit) 83. The SnO₂ ISFETand the reference electrode are then connected to the constantvoltage/current circuit 83, and immersed in the solution. Next, thedrain-source current IDS of the ISFET is set by adjusting the variableresistor R2 and measurement by one digital multimeter. Also, thedrain-source voltage V_(DS) is set at 0.2V by adjusting the variableresistor R1 and measuring with the other digital multimeter. Afterwards,the SnO₂ ISFET is immersed in the buffer solution for a period of time.Finally, the gate-source output voltage is recorded by the voltage-timerecorder, thereby obtaining the drift rate of the ISFET.

[0068] It should be noted that current generated by illumination affectsthe drift rate. Hence, the drain-source current should be adjustedbetween 10 μA and more than one hundred μA to reduce the illuminationeffect. As well, stability is easily affected by temperature when thedrain-source current I_(DS) is extremely large. Accordingly, thedrain-source current is preferably set at 10˜300 μA.

[0069] Table 4 shows the drift rates of the SnO₂ ISFET at pH 1-9, andFIG. 12 shows the relationship between the drift rate and the pH. Thedrift rate of the SnO₂ ISFET is obtained from the slope of the curvewhose time parameter is more than the fifth hour. TABLE 4 the driftrates of the SnO₂ ISFET at pH 1-9 pH Drift rate (mV/h) 1 0.11 2 1.07 32.28 4 3.52 5 4.28 6 5.35 7 6.73 8 7.37 9 8.15

[0070] It is believed that the drift behavior is more obvious when thepH is large. Also, when the data approximately form a line, the driftrates at any other pH can be estimated. This is useful when performingreverse compensation.

[0071] The method of forming the SnO₂ ISFET according to the presentinvention is simple, has a low cost and novel technology. The measuringmethods and apparatus can accurately measure hysteresis width and driftrate of the SnO₂ ISFET, and also hysteresis width and the drift rate ofthe ISFETs with other types of detection membranes.

[0072] While the invention has been described by way of example and interms of the preferred embodiments, it is to be understood that theinvention is not limited to the disclosed embodiments. To the contrary,it is intended to cover various modifications and similar arrangements(as would be apparent to those skilled in the art). Therefore, the scopeof the appended claims should be accorded the broadest interpretation toencompass all such modifications and similar arrangements.

What is claimed is:
 1. A SnO₂ gate ISFET device, comprising: a semiconductor substrate; a gate oxide layer on the semiconductor substrate; a SnO₂ layer overlying the gate oxide layer to form a SnO₂ gate; a source/drain in the semiconductor substrate beside the SnO₂ gate; a metal wire on the source/drain; and a sealing layer overlying the metal wire and exposing the SnO₂ layer.
 2. The SnO₂ gate ISFET device as claimed in claim 1, wherein the length of the channel, the width of the channel and ratio of width/length of the channel of the ISFET are 50 μm, 100 μm and 20, respectively.
 3. The SnO₂ gate ISFET device as claimed in claim 1, wherein the semiconductor substrate is p-type.
 4. The SnO₂ gate ISFET device as claimed in claim 1, wherein the resistivity of the semiconductor substrate ranges from 8 to 12Ω-cm.
 5. The SnO₂ gate ISFET device as claimed in claim 1, wherein the lattice parameter of the semiconductor is (1,0,0).
 6. The SnO₂ gate ISFET device as claimed in claim 1, wherein the thickness of the gate oxide layer is 1000 Å.
 7. The SnO₂ gate ISFET device as claimed in claim 1, wherein the metal wire is Al.
 8. The SnO₂ gate ISFET device as claimed in claim 1, wherein the sealing layer is epoxide resin.
 9. The SnO₂ gate ISFET device as claimed in claim 1, wherein the source/drain is N-type.
 10. A method for fabricating a SnO₂ gate ISFET device, comprising steps of: providing a semiconductor substrate; forming a virtual gate on the semiconductor substrate to define the gate area of the ISFET; forming a source/drain in the semiconductor substrate beside the virtual gate; removing the virtual gate; forming a SnO₂ gate in the gate area to form an ISFET.
 11. The method as claimed in claim 10, wherein forming the virtual gate to define the gate area of the ISFET further comprises: rinsing the semiconductor substrate; forming a pad oxide layer on the semiconductor substrate; and removing a portion of the oxide layer to form a virtual gate to define the gate area.
 12. The method as claimed in claim 11, wherein forming the SnO₂ gate in the gate area comprises: coating a solution comprising SnCl₂.2H₂O and ethanol on the surface of the gate oxide layer of the ISFET; and heating the semiconductor substrate to a predetermined temperature for a predetermined time interval.
 13. The method as claimed in claim 10, wherein forming the source/drain beside the virtual gate further comprises doping the semiconductor substrate by the virtual gate as a mask to form a source/drain.
 14. The method as claimed in claim 12, wherein the concentration of the solution comprising SnCl₂.2H₂O and ethanol is 0.37M.
 15. The method as claimed in claim 12, wherein the predetermined temperature ranges from 350° C. to 400° C.
 16. The method as claimed in claim 12, wherein the predetermined interval is one hour.
 17. The method as claimed in claim 1, wherein the thickness of the SnO₂ gate is at least 1000 Å.
 18. A method of measuring the temperature parameters of an ISFET with a SnO₂ as a detection membrane, comprising: contacting the detection membrane with a buffer solution; changing the pH of the buffer solution at a predetermined temperature; measuring and recording the source-drain current and the gate voltage of the ISFET to obtain a curve; selecting a fixed current from the curve to obtain the sensitivity of the ISFET at the predetermined temperature; changing the temperature of the buffer solution and repeating the steps of contacting, changing the pH, measuring, recording and selecting to obtain the sensitivities of the ISFET at different temperatures.
 19. The method as claimed in claim 18, wherein the increment of the gate voltage is caused by increasing per unit pH at the predetermined temperature.
 20. The method as claimed in claim 19, wherein the predetermined temperature is fixed by a temperature controller and a heater.
 21. The method as claimed in claim 20, wherein the predetermined temperature is between 15° C. and 45° C.
 22. The method as claimed in claim 21, wherein the pH of buffer solution is between 1 and
 10. 23. An apparatus for measuring the temperature of an ISFET with SnO₂ as a detection membrane, comprising: a semiconductor substrate where the ISFET is formed, comprising a pair of sources and drains separated from each other and the detection membrane insulated from the surface of the semiconductor substrate; a buffer solution contacting the ISFET; a light-isolating container for the buffer solution; a heater for the buffer solution; a temperature controller for the solution heater; a test fixer connected to the source and drain of the ISFET; and a current/voltage measuring device connected to the test fixer to measure and record the source-drain current and the gate voltage of the ISFET.
 24. The apparatus as claimed in claim 23, further comprising a reference electrode with one end contacting the buffer solution and the other end connected to the test fixer.
 25. The apparatus as claimed in claim 24, further comprising a thermometer with one end contacting the referring solution and the other end connected to the test fixer to detect the temperature of the referring solution.
 26. The apparatus as claimed in claim 24, wherein the detection membrane and the surface of the ISFET are isolated by a silicon oxide layer.
 27. The apparatus as claimed in claim 24, wherein the test fixer contacts the source/drain of the ISFET through an aluminum contact plug and an aluminum wire.
 28. The apparatus as claimed in claim 24, wherein the temperature controller is a PID temperature controller.
 29. A method of measuring the hysteresis width of an ISFET with SnO₂ as a detection membrane, comprising the steps of: fixing the drain-source current and the drain-source voltage of the ISFET by a constant voltage/current circuit; immersing the detection membrane in a buffer solution; recording the gate/source output voltage of the ISFET by a voltage-time recorder; and changing the pH of the buffer solution and repeating the step of fixing, immersing and recording to measure the hysteresis width of the ISFET.
 30. The method as claimed in claim 29, wherein the hysteresis width is the change in the gate/source output voltage from the first measuring point to the final measuring point.
 31. The method as claimed in claim 29, wherein the source-drain current is fixed at 50 μA, and the drain-source voltage is fixed at 0.2V.
 32. The method as claimed in claim 29, further comprising immersing the ISFET with SnO₂ as a detection membrane in a standard solution to maintain stability prior to immersing the detection membrane into the buffer solution.
 33. The method as claimed in claim 29, wherein the pH is changed from pH=4 to pH=1, to pH=4, to pH=7, and to pH=4.
 34. The method as claimed in claim 33, wherein each pH level of the buffer solution is fixed for one minute.
 35. A method of measuring the drift rate of an ISFET with SnO₂ as detection membrane (called SnO₂ ISFET), comprising: fixing the drain/source current and the drain/source voltage of the SnO₂ ISFET by a constant voltage/source circuit; immersing the detection membrane into a buffer solution; recording the gate/source output voltage of the SnO₂ ISFET during constant period by a voltage-time recorder to obtain the drift rate of the SnO₂ ISFET.
 36. The method as claimed in claim 35, further comprising a step of changing the pH of the buffer solution to measure the drift rates of the SnO₂ ISFET at different pH levels.
 37. The method as claimed in claim 36, wherein the drift rate is the change in the gate/source voltage per unit of time.
 38. The method as claimed in claim 36, wherein the gate/source current is fixed at 50 μA, and the drain-source voltage is fixed at 0.2V.
 39. The method as claimed in claim 35, further comprising a step of immersing the SnO₂ ISFET in a standard solution to maintain stability prior to immersing the SnO₂ ISFET in the buffer solution.
 40. The method as claimed in claim 37, wherein the gate/source output voltage of the SnO₂ ISFET is recorded for more than twelve hours.
 41. An apparatus of measuring the hysteresis width and the drift rate, comprising: a SnO₂ ISFET formed on a semiconductor substrate, comprising a pair of source/drain regions within the semiconductor and a detection membrane of SnO₂ isolated from the surface of the semiconductor substrate; a buffer solution for contacting the SnO₂ ISFET; a light-isolation container for isolating light and carrying buffer solution and the SnO₂ ISFET; a heater for heating the buffer solution; a constant current/voltage circuit coupled to the source and drain of the SnO₂ ISFET to fix the drain/source current and the drain/source voltage of the SnO₂ ISFET; a current/voltage measuring device coupled to the constant current/voltage circuit; and a voltage-time recorder coupled to the constant current/voltage circuit to record the gate/source output voltage of the SnO₂ ISFET.
 42. The apparatus as claimed in claim 41, further comprising a reference electrode with one end immersed in the buffer and the other end connected to the constant voltage/current circuit.
 43. The apparatus as claimed in claim 42, further comprising a thermometer with one end immersed in the buffer solution and the other end coupled to a temperature controller.
 44. The apparatus as claimed in claim 43, wherein the temperature controller fixes the temperature of the buffer solution at 25° C.
 45. The apparatus as claimed in claim 44, wherein the constant voltage/current circuit is a negative feedback circuit.
 46. The apparatus as claimed in claim 45, wherein the current/voltage measuring device is composed of digital multimeters.
 47. The apparatus as claimed in claim 46, wherein the constant voltage/current circuit is connected to the source/drain of the SnO₂ ISFET by an aluminum contact plug and an aluminum wire. 